Pulse photometry is a noninvasive technique for measuring blood analytes in living tissue. In this technique, multiple light sources emit light of differing wavelengths, which is transmitted through or reflected from a vascular bed. One or more photodetectors then detect the transmitted or reflected light as an optical signal. As the photons propagate through the tissue, they are subjected to random absorption and scattering processes due to the nonhomogeneous nature of the tissue. These effects manifest themselves as a loss of energy in the optical signal, and are generally referred to as bulk loss. In addition to bulk loss, the optical signal is modulated by the flow of arterial blood into the vascular bed. Moreover, the movement of venous blood into or out of the tissue, local tissue compression and local muscle movements super-impose yet another modulation on the optical signal, usually of lower frequency than the arterial flow. For example, FIG. 1 illustrates detected optical signals that include the foregoing attenuation, arterial flow modulation, and low frequency modulation. Each optical signal, with its combined attenuation and modulations, e.g., each combined optical signal, is generally referred to as a photoplethysmograph (photopleth.)
Pulse oximetry is a special case of pulse photometry where the oxygenation of arterial blood is sought in order to estimate the state of oxygen exchange in the body. In order to calculate the oxygen saturation of arterial blood, two wavelengths of light, e.g. Red, at about 660 nm, and Infrared, at about 900 nm, are used to calculate the ratio of two dominant hemoglobin components, oxygenated hemoglobin (HBO2) and deoxygenated hemoglobin (HB). The detected optical signals, which correspond to the Red and Infrared wavelengths, are first normalized in order to balance the effects of unknown source intensity as well as unknown bulk loss at each wavelength. The arterial pulses are then isolated by filtering each normalized signal, where a high pass or a band-pass filter takes advantage of the typically higher frequency of the pulsatile arterial blood, hence the name pulse oximetry. This normalized and filtered signal is referred to as the AC component and is typically sampled with the help of an analog to digital converter with a rate of about 30 to about 100 samples/second. For example, FIG. 2 illustrates the optical signals of FIG. 1 after they have been normalized and bandpassed.
In order to estimate blood oxygenation, a (Red/Infrared) ratio is calculated by dividing the strength of the Red AC (RdAC) by the corresponding strength of the Infrared AC (IrAC). The (RdAC/IrAC) ratio is then generally plugged into an empirical calibration curve equation that relates it to blood oxygenation. For example, reference can be made to Japanese Patent No. Sho 50/1975-128387, issued to Aoyagi, entitled “Optical Type Blood Measuring Equipment.”
The arterial blood flow generally has a higher fundamental frequency than other components of the photopleth, however, there are cases where the two frequencies may overlap. One such example is the effect of motion artifacts on the optical signal, which is described in detail in U.S. Pat. No. 6,157,850, issued to Diab et al., entitled “Signal Processing Apparatus.” Another effect occurs whenever the venous component of the blood is strongly coupled, mechanically, with the arterial component. This condition leads to a venous modulation of the optical signal that has the same or similar frequency as the arterial one. Such conditions are generally difficult to effectively process because of the overlapping effects.
As described in the Aoyagi patent, the strength of each AC waveform may be estimated by measuring its size through, for example, a peak-to-valley subtraction, by a root mean square (RMS) calculations, integrating the area under the waveform, or the like. These calculations are generally least averaged over one or more arterial pulses. It is desirable, however, to calculate instantaneous ratios (RdAC/IrAC) that can be mapped into corresponding instantaneous saturation values, based on the sampling rate of the photopleth. However, such calculations are problematic as the AC signal nears a zero-crossing where the signal to noise ratio (SNR) drops significantly. For example, dividing two signals with low SNR values can render the calculated ratio unreliable, or worse, can render the calculated ratio undefined, such as when a near zero-crossing area causes division by or near zero. To try to avoid division by zero, the Ohmeda Biox pulse oximeter calculated the small changes between consecutive sampling points of each photopleth in order to get instantaneous saturation values. FIG. 3 illustrates various techniques used to try to avoid the foregoing drawbacks related to zero or near zero-crossing, including the differential technique attempted by the Ohmeda Biox.
Note that Ohmeda's differential technique is equivalent to a calculation over a derivative of the photopleth, and the derivative has the same low SNR problem whenever a flattened section of the photopleth is used in the ratios calculations. For example, the derivative will have a zero or near zero value and the RdAC/IrAC ratio will become unreliable or undefined, even in a substantially noise free signal. For example, FIG. 4 illustrates the derivative of the IrAC photopleth plotted along with the photopleth itself. As shown in FIG. 4, the derivative is even more prone to zero-crossing than the original photopleth as it crosses the zero line more often. Also, as mentioned, the derivative of a signal is often very sensitive to electronic noise. For example, according to “Pulse Oximetry: Analysis of Theory, Technology, and Practice,” Journal of Clinical Monitoring, Vol. 4, October 1988, a published paper by the designers of the Ohmeda Biox, the calculated instantaneous saturations over some sections of the photopleth can be off by more than 50 percent (0/0) from the real value over a time as short as 1/10th of a second. As the designers described in their paper, this result is clearly an artifact of the signal processing technique employed in the Biox pulse oximeter since the blood saturation value can not change by that amount in 1/10th of a second.
Because of some of the foregoing drawbacks associated with the determination of instantaneous or point-by-point saturation from RdAC/IrAC ratios, designers now typically unequally weigh the calculated instantaneous saturation values over each photopleth, even when the photopleth is substantially noise free, with the consequence that a significant number of saturation values receive insignificant weights. This is tantamount to filtering out or ignoring valid signal data during the troublesome sections described above.